Limited slip differential having thermal compensating valve for regulating torque bias

ABSTRACT

A limited slip differential for an automotive vehicle includes a torque input differential pinion carrier, two differential side gears and a pair of torque output axle half shafts. The differential also includes at least one friction clutch assembly having first friction clutch plates connected driveably to the differential pinion carrier and second friction clutch plates connected driveably to one of the side gears. An annular pressure chamber is defined in the carrier. An annular piston is movably supported within the pressure chamber and adjacent the friction plates of the friction clutch assembly. The limited slip differential further includes a positive displacement gear pump having pumping elements defining high pressure and low pressure ports whereby pressure in the pressure chamber creates a force on the piston that engages a clutch assembly to effect a friction torque bias in the differential through the friction clutch assembly. The piston includes at least one flow control orifice through which fluid may flow from the high pressure port of the positive displacement gear pump via the annular pressure chamber and a thermal compensating valve that is mounted on the piston and is operable to decrease the flow area of the flow control orifice upon increasing temperature. Furthermore, the thermal compensating valve is also operable to increase the flow area of the flow control orifice upon decreasing temperature.

This application is a continuation of U.S. Ser. No. 09/264,282, filed onMar. 8, 1999, U.S. Pat. No. 6,120,408, issued Sep. 19, 2000.

TECHNICAL FIELD

The invention relates to a limited slip differential for an automotivevehicle and, more specifically, to a limited slip differential having athermal compensating valve which regulates torque bias in thedifferential.

BACKGROUND OF THE INVENTION

In applications that require a relatively uniform flow of pressurizedfluid to a pressure-operated mechanism, such as a friction clutch, theactuating pressure typically is calibrated for operation within apredetermined temperature range. If the operating temperature of thefluid decreases to a low level outside the calibrated temperature range,the resulting fluid viscosity increase may adversely affect performanceof the pressure-actuated mechanism. Conversely, if the operatingtemperature is higher than the calibrated temperature range, viscositydecreases in the hydraulic fluid may adversely affect performance.

If the pressure-actuated mechanism is a friction clutch in a limitedslip differential mechanism for an automotive vehicle driveline, theclutch transfers torque between a differential side gear of thedifferential mechanism and a differential pinion carrier. To compensatefor viscosity changes, it is necessary to account for temperaturechanges in the hydraulic fluid pressure delivered to a pressure chamberof the friction clutch. In the case of a limited slip differentialmechanism for a rear wheel drive vehicle, the differential side gearthrust force may be relied upon to apply the clutch. The torque bias inthe differential mechanism developed by the clutch during lowtemperature operation should emulate the torque bias that would normallyexist at warmer temperatures for which the differential mechanism iscalibrated.

Viscosity change compensation for the pressure fluid is needed to ensurereliable operation of the limited slip differential. A lack of viscositychange compensation could cause premature application of the clutch. Ina limited slip differential for a rear wheel drive vehicle, this maycontribute to unpredictable handling of the vehicle or cause so-called“crow hopping” of the vehicle during steering maneuvers. In the case ofa limited slip differential for a front wheel drive vehicle, a lack ofviscosity change compensation may tend to cause an“under-steer”condition during steering maneuvers.

The side gears for a differential mechanism of this kind engagedifferential pinions that in turn are journalled on a pinion shaft orspider member that is supported by a differential carrier, the carrierin turn being driven by a crown gear. The side gears are connectedrespectively to each of two axle half-shafts for the vehicle tractionwheels. Examples of differential mechanisms of this kind may be seen inU.S. Pat. Nos. 5,536,215, 5,595,214, 5,310,388, and 5,611,746, which areassigned to the assignee of this invention. Their disclosures areincorporated herein by reference.

The limited slip differentials disclosed in these reference patentsinclude a speed sensitive torque bias wherein the bias torque is relatedto the difference in the speeds of the differential side gears and thepinion carrier. A lack of viscosity change compensation may adverselyaffect the speed sensitive bias as well as the torque sensitive bias.

DISCLOSURE OF INVENTION

The present invention is directed toward a limited slip differential foran automotive vehicle including a torque input differential pinioncarrier, two differential side gears and a pair of torque output axlehalf shafts. The differential also includes at least one friction clutchassembly having first friction clutch plates connected driveably to thedifferential pinion carrier and second friction clutch plates connecteddriveably to one of the side gears. An annular pressure chamber isdefined in the carrier. An annular piston is movably supported withinthe pressure chamber and adjacent the friction plates of the frictionclutch assembly. The limited slip differential further includes apositive displacement gear pump having pumping elements defining highpressure and low pressure ports whereby pressure in the pressure chambercreates a force on the piston that engages a clutch assembly to effect afriction torque bias in the differential through the friction clutchassembly. Furthermore, the piston includes at least one flow controlorifice through which fluid may flow from the high pressure port of thepositive displacement gear pump via the annular pressure chamber and athermal compensating valve that is mounted on the piston and is operableto decrease the flow area of the flow control orifice upon increasingtemperature. Furthermore, the thermal compensating valve is alsooperable to increase the flow area of the flow control orifice upondecreasing temperature. In this way, the operation of the thermalcompensating valve controls the valve of pumped hydraulic fluid from thepressure chamber and thereby controls the torque bias in thedifferential.

The thermal compensating valve in a limited slip differentialenvironment may also include a second flow control valve in the frictionclutch pressure chamber for independently controlling flow ofpressurized actuating fluid for the clutch. In the preferred embodimentthe thermal compensating valve may include a slide valve plate whichslidably engages the base plate and is movable linearly relative to abase plate. Guide edges on the base plate are positioned on oppositesides of the slide valve plate. The slide valve plate has a flowmetering edge positioned over the base orifice so that the effectivefluid flow area of the orifice is varied as the actuator coiltemperature changes. The valve on the piston provides a fluid flow pathfrom the clutch pressure chamber to the low pressure fluid flow returncircuit for the positive-displacement pump.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a limited slip differential assemblycapable of embodying the valve of the present invention;

FIG. 1a is a cross-sectional view of a one-way check valve located atthe flow inlet port for a positive-displacement pump, which is a part ofthe differential assembly of FIG. 1;

FIG. 2 is a cross-sectional view of a positive-displacement pump, asseen from the plane of section line 2—2 of FIG. 1;

FIG. 3 is a cross-sectional view of the base of a temperaturecompensating valve assembly of the invention taken along the plane ofsection line 3—3 of FIG. 3a;

FIG. 3a is a plan view of the base of FIG. 3;

FIG. 3b is an end view of the base of FIG. 3 as seen from the plane ofsection line 3 b—3 b of FIG. 3;

FIG. 4 is a subassembly plan view of a bimetallic coil and slide valveplate which form a part of the temperature compensating valve of theinvention;

FIG. 4a is a subassembly side view of the bimetallic coil and slidevalve plate seen in FIG. 4;

FIG. 4b is a cross-sectional view taken along the plane of section line4 b—4 b of FIG. 4;

FIG. 5 is a cross-sectional view of a cover that is connected to thebase of FIG. 3. It is taken along the plane of section line 5—5 of FIG.6;

FIG. 6 is a plan view of the cover shown in FIG. 5; and

FIG. 7 is a plan view of a second embodiment of the valve of the presentinvention in combination with a clutch actuating piston used in thedifferential mechanism of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiment of the invention will be described withreference to a limited slip differential of the type shown in FIG. 1.More specifically, a geared differential shown in FIG. 1 is designed fora rear-wheel drive vehicle. It provides both torque sensitivity andspeed sensitivity to effect a total torque bias that is proportionalboth to the relative speed of the differential carrier and side gearsand to the torque transferred to the axle shafts.

The differential carrier includes a carrier housing 10, which isjournalled by a roller thrust bearing 12 at the right-hand side of thedifferential assembly in an axle housing 14. A carrier housing coverplate 16 is secured by bolts 18 to the left-hand side of thedifferential carrier housing 10. End plate 16 defines a pump cavity thatreceives a positive-displacement pump 20, which will be described withreference to FIG. 2. Pump 20 includes a first pumping gear element 22with external teeth and a second pumping gear element 24 with internalteeth.

As seen in FIG. 2, the first gear element 22 has external teeth thatmesh with internal gear tooth spaces formed in second pumping element24. The number of external teeth in element 22 is one less than thenumber of tooth spaces in second pumping gear element 24. The centralaxis of pumping gear element 24 is offset relative to the axis ofpumping gear element 22 so that the tooth spaces define pumping chambersthat expand and contract as one pumping element rotates relative to theother. First pumping gear element 22 is splined or keyed at 26 to axleshaft 28.

An annular cylinder member 30 is situated directly adjacentpositive-displacement pump 20 and is secured by bolts 29 to the plate 16so that it rotates with the carrier housing 10. An annular piston 32 isreceived in the cylinder member 30 to define a clutch pressure chamber34.

A clutch hub 36 is splined to the axle shaft 28. A friction disk clutchassembly 38 is positioned directly adjacent piston 32. It includesexternally splined clutch disks that are connected driveably to thecarrier housing 10. Cooperating internally splined clutch disks ofclutch assembly 38 are splined to hub 36.

A first side gear 40 and a second side gear 42 driveably engagedifferential pinions 45 journalled on pinion shaft or spider member 46.Pinion shaft or spider member 46 is connected driveably to carrierhousing 10. Side gear 42 is splined to a second axle shaft 44. The axleshafts 28 and 44 are connected to vehicle traction wheels.

A seal cover plate 48 located adjacent the outboard side of cover plate16 defines a fluid intake cavity 50, which communicates with theinterior of the axle housing 14. Axle housing 14 serves as a reservoirfor axle fluid as seen at 52. The reservoir 52 communicates with cavity50 through fluid intake tube 54.

First and second pump intake ports, shown respectively at 56 and 58, areformed in the cover 16. They are located on diametrically opposite sidesof the axis of the gear pumping element 22. They are angularly disposedso that they communicate with the gear tooth spaces when the gear toothspaces are increasing in volume as pumping element 22 and pumpingelement 24 rotate, one with respect to the other.

When the pumping elements 22 and 24 rotate relative to each other in onedirection, intake port 56 receives fluid from reservoir 52. When thedirection of relative rotation of the pumping elements 22 and 24 isreversed, fluid is received from reservoir 52 by intake port 58.

A one-way check valve plate 60 provides one-way fluid flow from thecavity 50 to the intake port 56. Similarly, one-way check valve plate 62provides one-way flow from cavity 50 to the intake port 58. The valvesare mounted on slide pins 61 secured to the plate 16, as seen in FIG.1a. These pins accommodate movement of the valve plates between valveport opening and closing positions.

The end plate 16 is journalled by roller thrust bearing 64 in the axlehousing 14. Axle shaft 28 is rotatably journalled in plate 16 by bushing66. The carrier housing 10 is driven by a ring gear or crown gear 68,which meshes with a drive pinion (not shown).

A thrust ring 70 is located directly adjacent the clutch assembly 38. Ithas an internal cone surface that is engaged by an external cone surfaceon the side gear 40. Thrust force from the side gear 40 then istransmitted directly to the friction discs of the clutch 38, thusestablishing a friction torque bias in the differential assembly. Thereaction point for the thrust forces is the cylinder member 30, which isengaged by annular piston 32.

Pump 20 has pressure ports that communicate with gear tooth spaces ofthe pumping gear elements 22 and 24. They are positioned ondiametrically opposite sides of the axis of the pumping element 22 andare angularly positioned at a location coinciding with the pumpingelement positions where the gear tooth spaces are decreasing in volumeas the pumping elements rotate, one with respect to the other.

The pump outlet ports are formed in cylinder member 30. They areindicated at 72 and 74, respectively. When the pumping elements rotaterelative to one another in one direction, port 72 is pressurized. Whenthey rotate relative to one another in the opposite direction, port 74is pressurized. One-way outlet check valve plates for the pump arelocated at each outlet port 72 and 74, as indicated at 76 and 78,respectively. The valves 76 and 78 are normally closed. One of theoutlet check valves is opened when the relative rotation of the pumpinggear elements is in one direction, and the other outlet check valveopens when the relative rotation is in the opposite direction. Thisprevents recirculation of the pumped fluid through the pump.

The piston 32 is provided with a valve opening, which is controlled by apoppet control valve 80. The valve 80 delays application of the clutchas the pumping elements begin to rotate because it initially is open. Asdifferential speed of the side gears increases, the flow through theopening of valve 80 increases. The flow then will cause the valve toclose, thereby allowing pressure to build up in the pressure chamber 34.

Check valves 76 and 78 are generally similar in construction to thevalve 80 although they assume a normally closed position rather than anormally open position. When one of the valves 76 and 78 is opened, theother is closed. As previously described, when relative rotation of thepumping elements is in one direction, one outlet valve element is closedand the other is opened. When the relative rotation is reversed, thevalve plate positions are reversed.

Valve 80 has a bleed orifice (not shown) which bypasses the valve andprovides a restricted, continuous flow of fluid past the valve whileallowing the pressure buildup to occur in the clutch pressure chamber.This helps to avoid a buildup of contaminants. It also allows thepressure in the clutch pressure chamber to bleed down, thereby allowingthe valve to reopen when differential motion of the pinon carrier andthe side gears decreases.

Clutch piston 32 is illustrated in FIG. 7, which is a side view of thepiston. The piston is provided with a flow control orifice 92, whichpermits a flow of fluid from the pressure chamber 34 to the flow returncircuit leading to the axle housing (not shown), which acts as a fluidreservoir. The flow through orifice 92 is controlled by the thermalcompensating valve of the present invention. The thermal compensatingvalve is mounted on the piston 32 and operable to decrease the flow areaof the flow control orifices upon increasing temperature and that isoperable to increase the flow area of said opening upon decreasingtemperature to control the flow of pumped hydraulic fluid from thepressure chamber and thereby control the torque bias in thedifferential.

One embodiment of the valve is indicated generally in FIG. 7 byreference numeral 94. It includes a base plate 96, which has a reducedwidth portion 98 and a reduced width portion 100.

The base plate 96 has a pair of guide tabs 102 at reduced width portion98 and a pair of guide tabs 104 at reduced width portion 100. Thereduced width portion 100 has an end tab 106 to which one end of athermostatic, double-wound helix element is attached. The double-woundhelix element, which will be described subsequently, has an end that isreceived through an opening in end tab 106. The helix element end isbent at the outboard side of tab 106 so that the helix element isanchored by tab 106 while allowing twisting movement of the end.

The base plate 96 is secured to the piston 32 at three locations 108,110 and 112. Suitable fasteners, such as rivets, can be provided forthis purpose.

A slide valve plate 113 is situated directly adjacent the surface ofbase plate 96. It is mounted in place by a pair of tabs 116, which arestamped from the base plate 96. The spacing between the tabs 116 isslightly greater than the width of the sliding valve plate 113 so thatthe valve plate 113 can be freely adjusted fore-and-aft in a lineardirection relative to the base plate 96 while being restrained fromrelative lateral movement.

In the embodiment shown in FIG. 7, the base plate is formed with atriangular orifice 118 which registers with the opening 92. One end 121of a double-wound bimetallic coil 123 is secured to slide plate 113, asshown at 125. A spot welding technique may be used if desired toestablish a connection between the end 121 and the slide valve plate113. The leading edge of the valve plate slides over the orifice 118 tovary the effective flow area. In the alternative, a triangular orificemay be formed in the slide valve plate 113 and one edge of the orificemay slide over an opening in the base plate to effect a variable flowarea.

FIGS. 4, 4 a and 4 b illustrate a bimetallic coil and slide valve platesubassembly for a second embodiment that will be described withreference to FIGS. 3, 3 a and 3 b. The coil and slide valve platesubassembly for the embodiment of FIG. 7 is essentially similar,however, to that illustrated in FIGS. 4, 4 a and 4 b. Their functionsare the same.

The double-wound bimetallic element 122 of FIGS. 4, 4 a and 4 bcomprises a bimetallic strip (ASTM type TM2). The bimetallic stripcomprises a first metal layer 126 and an adjacent metal layer 128 whichare bonded together in known fashion. The bimetallic strip is firstwound in a tight helix. The helix then is wound to form a helix oflarger diameter, as indicated at 130. The large diameter helix 130resembles a coil spring having multiple coils. The left end 132 of thebimetallic element forms a hook 134, which engages the base. This willbe described subsequently.

Because of the differential coefficients of thermal expansion of themetal 126 with respect to the metal 128, the normal tendency of thebimetallic element to assume a bowed shape, when the element is in itsunwound state, establishes a linear motion of the end 120 with respectto the anchored end in response to temperature changes. The end 120 maybe secured to slide valve plate 114 by welding, as shown at 124. Whenthe temperature of the fluid drops, the double-wound helix bimetalliccoil retracts, thereby drawing the slide valve plate 114 to the left.This increases the effective size of the flow control orifice. Theorifice acts as a pressure dump from the pressure chamber behind thepiston 90 and prevents the clutch 38 from engaging prematurely at coldtemperatures due to the increased fluid viscosity.

FIG. 4a shows a side view of the thermal, bimetallic, double-wound helixelement, mentioned earlier, corresponding to the bimetallic coil andslide valve plate assembly shown in FIG. 7. The coil construction ofFIG. 4a is the same as the coil construction of FIG. 7, although the end132, as mentioned previously, is formed with a U-shaped hook 134 that isadapted to be received in an end opening in a molded housing that willbe described subsequently. This provides an anchor for the end 132 ofthe coil, designated by reference character 122. The right-hand end ofthe coil 122 is secured, such as by welding, to the slide valve plate114.

FIGS. 3, 3 a, 3 b, 4 and 4 b show an embodiment, mentioned previously,for the thermal compensating valve assembly of the invention. This isthe preferred embodiment. Unlike the design seen in FIG. 7, which has asteel base 96, the preferred construction of FIGS. 3-4 comprises aplastic or phenolic base, seen at 138. The material may be a resin,preferably polyetherimide (Ultem 1000, manufactured by the G.E. PlasticsDivision of General Electric Company). The right and left ends of base138 are provided with attachment openings 140 and 142, which receiverivets or some other suitable fastener device for securing the base 138to the piston 32. The base 138 is provided with a flow opening 144,which corresponds to opening 92 seen in FIG. 7.

Base 138 has guide edges 138′ and 138″, which guide the linear motion ofslide valve plate 114. As in the case of the embodiment of FIG. 7, wherethe valve plate 113 has a sliding clearance with respect to tabs 116,there is a sliding clearance between valve plate 114 and guide edges138′ and 138″.

The base opening 144, which preferably is triangular, as seen in phantomin FIG. 4, registers with a similarly shaped opening 118′ in slide valveplate 114. The edge of the opening 118′ moves over the opening 144 asthe temperature of the bimetallic coil 122 changes.

The left end of a double-wound, helix, bimetallic coil of the type shownin FIGS. 4 and 4a is secured to a peripheral wall 146, thereby providingan anchor for the left end of the coil. The wall 146 has a half-roundopening 147 at its left end, which receives the U-shaped hook 134 of thebimetallic coil end 132. The end 132 is not restrained from twistingmotion, although it is restrained from linear motion during operationwhen the coil temperature changes.

The valve assembly includes further a cover 148, seen in FIGS. 5 and 6.The cover 148 defines a chamber that encloses the bimetallic coil andthe slide valve element. Cover 148 has a peripheral wall 150 thatregisters with the corresponding peripheral wall 146 for base 138. Thecover 148 is formed of the same material as the base 138. The cover 148and the base 138 may be fastened together by ultrasonic welding, bysolvent bonding or by an integrated “snap” feature.

The wall 150 has a half-round opening 149, which registers withhalf-round opening 147 in base 138. When assembled, the hook 134 is heldin the full rounded opening defined by the half-round openings 147 and149. Any slight twisting motion of the bimetallic coil end 132 isaccommodated.

The cover 148 is provided with an opening 152 which is aligned with theopening 144 in the base 138. A guide surface 153 on cover 148 surroundsopening 152. It positions the slide valve plate 114 for linear, slidingmovement. There is sufficient clearance between valve plate 114 andsurface 153 to avoid binding.

In the preferred embodiment shown in FIGS. 3, 3 a, 5 and 6, the fluid ispermitted to flow around the bimetallic coil. This flow is improved byforming a plurality of openings in the peripheries of cove 148 and base138. The openings may be formed by half-round recesses 151 in theperipheral wall 146 for base 138 and by half-round recesses 155 in theperipheral wall 150 for the cover 148. When the cover and the base areassembled together, recesses 151 register with recesses 155 to definefull openings.

The characteristic curve in a plot of the effective area of the flowcontrolling orifice as a function of temperature of the coil can bevaried depending upon the geometry of the orifice formed in the base. Inthe case of the embodiments shown in FIGS. 3, 3 a and 7, the effectiveorifice size will vary nonlinearly. The shape of the characteristiccurve, however, may be changed by changing the shape of the orificedepending upon the requirements of the particular differential design orthe requirements of other applications for which the invention issuited. Experiments have shown that there is more motion of the movablevalve plate and more valve plate actuating force for a given packagesize than other types of bimetallic valves.

The bimetallic coil is constructed using long, flat strip stockbimetallic material. The bimetallic coil delivers the maximum linearmotion and linear force for its package size. The normal tendency of abimetallic strip to assume a bowed condition in its pre-wound state dueto a change in temperature is transformed into a linear motion of theslide valve element. As previously explained, this compensates forchanges in the fluid viscosity due to temperature changes.

The change in the viscosity of a fluid usually is a nonlinear functionof temperature change. That is true, for example, for axle lube fluidscommonly used with automotive vehicles. Since the length of thebimetallic coil changes linearly over its normal operating range, theorifice shape can be tailored to establish the correct relationshipbetween the linear motion of the slide and the nonlinear viscositychanges of the hydraulic fluid. Moving the slide across a triangularshaped flow control orifice, for example, will have the effect ofopening the orifice area in an exponential fashion. Other orifice shapescan be used depending upon the fine tuning requirements of a particularapplication.

The thermal compensating valve performs reliably in an application suchas an automotive differential environment, notwithstanding the hydraulicpressures to which the slide valve is exposed. Those pressures mayexceed, for example, 1000 psi. Further, the valve will function reliablythroughout a wide temperature range. For example, the temperature rangein an automotive differential or transmission environment might be −40°to 300° F.

The thermal compensating valve further will resist leakage across theorifice when it is in its closed state during operation with high fluidtemperatures. It also is capable of withstanding constant oil emersionwithout deterioration.

The bimetallic coil valve assembly of the preferred embodiment makes itpossible for the limited slip differential of FIG. 1, for example, tooperate during cold temperatures by dumping hydraulic fluid, therebyemulating the performance during operation at warm temperatures at whichthe poppet control valve calibration is optimized. Premature closing ofthe piston valve 80, in the case of the assembly of FIG. 1, can beavoided. This improves the predictability of the function of thedifferential.

Although preferred embodiments of the invention have been described,modifications to the invention may be apparent to a person skilled inthe art without departing from the scope of the invention. All suchmodifications and equivalents thereof are covered by the followingclaims.

What is claimed is:
 1. A limited slip differential for an automotivevehicle, said limited slip differential comprising: a torque inputdifferential pinion carrier, two differential side gears and a pair oftorque output axel half-shafts; at least one friction clutch assemblyincluding first clutch friction plates connected drivably to thedifferential pinion carrier and second clutch friction plates connecteddrivably to one of said side gears; an annular pressure chamber definedin said carrier, an annular piston movably supported within saidpressure chamber and adjacent the friction plates of said frictionclutch assembly; a positive displacement gear pump having pumpingelements defining high pressure and low pressure ports whereby pressurein said pressure chamber creates a force on said piston that engagessaid clutch assembly to effect a friction torque bias in thedifferential through said friction clutch assembly; said pistonincluding at least one flow control orifice through which fluid may flowfrom said high pressure port of said positive displacement gear pump viasaid annular pressure chamber and a thermal compensating valve that ismounted on said piston, said thermal compensating valve including aslide valve plate movable linearly relative to said piston and having aflow metering edge which is positioned over the flow control orifice insaid piston such that movement of said slide valve plate changes theeffective fluid flow area of said orifice as a function of thetemperature of the hydraulic fluid so that said thermal compensatingvalve is operable to decrease the flow area of said flow control orificeupon increasing temperature and is operable to increase the flow area ofsaid flow control orifice upon decreasing temperature to control theflow of pumped hydraulic fluid from said pressure chamber and therebycontrol the torque bias in said differential.
 2. A limited slipdifferential as set forth in claim 1 wherein said piston includes guidespositioned on opposite sides of said slide valve plate to facilitaterectilinear movement of said slide valve plate relative to said piston.3. A limited slip differential as set forth in claim 2 wherein saidslide valve plate includes an opening that registers with said fluidflow control orifice such that rectilinear movement of said slide valveplate as a function of the temperature of the hydraulic fluid increasesor decreases the amount of registration between said opening and saidfluid flow control orifice.
 4. A limited slip differential as set forthin claim 3 wherein said opening has a margin that defines said flowmetering edge, said flow metering edge being generally linear andextending transversely across said fluid flow control orifice in anoblique direction relative to the direction of the rectilinear movementof said slide valve plate.
 5. A limited slip differential as set forthin claim 4 wherein said flow metering edge of said opening in said slidevalve plate defines at least one side of a triangle.
 6. A limited slipdifferential as set forth in claim 5 wherein said slide valve platefurther includes a temperature sensitive actuator having a pair of endswith one end secured to said piston and the other end secured to saidslide valve plate to impart rectilinear movement to said slide valveplate in response to changes in temperature of said hydraulic fluid. 7.A limited slip differential as set forth in claim 6 wherein saidtemperature sensitive actuator includes a bi-metallic, double-wound coilhaving first and second ends.
 8. A limited slip differential as setforth in claim 7 wherein said actuator coil comprises a bimetallic stripwound in a first helical configuration with a first coiled diameter endalso being wound in a second helical configuration with a second coileddiameter greater than the first coiled diameter whereby said slide valvemoves linearly as a function of changes in temperature of said valve. 9.A limited slip differential as set forth in claim 8 wherein theeffective size of said fluid flow control orifice increases as thetemperature of the bi-metallic actuator coil decreases therebycompensating for increases in viscosity of fluid flowing through theflow control orifice.